An optical cavity may be any region bounded by two or more reflective interfaces that are aligned to provide multiple reflections of light waves. Optical cavities have been monitored or measured using a single wavelength illumination source such as a helium/neon (HeNe) laser. A change in the cavity size is detected by observing the change in reflected or transmitted intensity at the single wavelength. Monitoring a single wavelength reflected intensity requires a much larger signal-to-noise ratio (S/N) than a broadband technique. In certain applications, the cavity is filled with a fluid that is designed to have an index of refraction as close as possible to that of the bounding surfaces, making the reflectivity very small. In such cases, the S/N will be small and thus may not be sufficient to use a single wavelength technique. In a single wavelength system, there is not a one-to-one correspondence between the measured intensity to the optical path length; in other words, a measured intensity may correspond to any number of optical path lengths. Therefore, a single-wavelengh system cannot determine the absolute value of the optical path length, it can only detect changes. The change in optical path length as measured by a single-wavelengh system is sufficient for some servo applications where the path length is to be held constant; however, this allows the possibility of mode-hopping where the servo unintentionally and undesirably locks onto a different spectral mode.
Therefore, there is a need for an apparatus and method for measuring or monitoring an optical cavity path length with an output that provides the absolute value of the optical path length, has better S/N tolerance, is free of mode-hopping limitations, and offers near real time operation.